Heat engines using air, steam, mixtures of air and steam, and other working media have been used for over a century, and most of these use a single gaseous fluid as a working medium. The steam engine, especially the steam turbine, has been the most popular and successful heat engine, and present commercial steam engines have maximum efficiencies of less than 40% in converting the energy available from fuel into shaft work. Steam engines and other workable heat engines have used an external heat sink, either by direct discharge to the environment in an open cycle or to a condenser for a closed steam cycle system. It is not necessary to use a condenser to reject this latent heat to the environment. Under certain conditions, the latent heat of the prime mover can be transferred to a different portion of the system.
The temperature of compressed air discharged from an air motor after having accomplished work is very cold because heat had been extracted in the form of work. This observation led to the conclusions, based on calculations of state, that it would be possible to extract sufficient energy in the form of mechanical work from a system loaded near the stall point so that all the vapor would be liquefied in the prime mover. This is equally applicable to a piston motor, vane motor or turbine.
The range of operation is, however, extremely narrow and operation outside the range in either over-loaded or insufficiently loaded conditions will cause the unit to shut down. In this mode of operation, all the available heat energy is transferred to the mechanical work portion of the process.
With the prime mover properly loaded, the heat of condensation is available to perform shaft work. This is especially true and has been observed in piston, vane and impulse turbines, and it is a condition to be avoided in reaction turbines and in most vane-type impulse turbines to prevent their destruction. Terry turbines will typically condense 70 to 80 per cent of the vapor.
While the above-described process can produce mechanical work, much energy would be lost in the gearing required to increase the rotational speed of the output shaft. A methodology exists, however, to extend the range of operation of the prime mover from the stall point to where useable amounts of shaft work can be extracted.